The faults which occur during the manufacture and finishing of textile webs are not completely avoidable. At the Textile Mill Colloquium, held in Denkendorf on Oct. 20-21, 1993, G. Besenreuther asserted that even today a textile web with a length of 120 meters, which exhibits from 12 to 15 faults, is considered the first choice. The fault number refers to a web coming from the mill, as supplied to a finisher for further processing. From the ready-made goods industry comes the requirement, however, that the number of faults for such a web must be reduced to less than 10.
The greater part of the faults arises during production of the web structure. The machine must be stopped immediately, if it produces a fault. This has already been achieved through extensive monitoring equipment: warp-thread monitors and woof checking during weaving, monitoring of the course of the yarn and needles in the case of knitting as well as the halting of the machine with an optical alarm belong to the state of the art. The machine operators remove the fault at the machine. The fault in the textile web can as a rule be removed only later, however, because it at the moment is not located at a point in the machine accessible to interventions. Although the information that a fault has occurred as well as its position are known, this information is however lost in the case of present-day manufacturing processes. Sometimes corresponding marking threads are of course inserted at those positions which stopped the machine, thus making finding them again easier.
In a special testing step, the so-called goods inspection, the web is checked by eye over its entire length on the show table. The faults are thereby removed insofar as possible, for example, by removing thread ends, trimming away knots, cleaning up stains and the like. The search for faults on the web imposes great demands upon the attention of the personnel.
A system is marketed under the name Wisotex by the firm, Zellweger AG, to automate the search for faults. This system includes a video camera with optical image recognition, which marks the faults discovered. The marked positions are then checked during the goods inspection and removed insofar as possible. All the markings are then removed, and the corresponding information is thus lost. The markings must be removed, however, in order not to cause later faults during the finishing of the web. Due to the high cost factor and deficient reliability, this system has not however been successful in the marketplace. This is not astounding, if one is acquainted with the multitude of possible web-fault types which can occur. Reference is made in this context to the Katalog der Gewebefehlerarten im Rohgewebe Catalogue of the Types of Web Faults in the Raw Weave!, published by the International Textile Service, Ltd., Schlieren, Switzerland, 1989.
U.S. Pat. No. 4,146,061 (Nissan Motor Corporation) is concerned with the faults occurring at the loom. The detection of faults is limited to breaks in the warp and fill threads, which are determined by the loom itself. In addition, length markings are woven into the web edge, which are based upon the absolute length measurement. The resulting data can be reproducibly employed during the first goods inspection and are thereafter lost.
In the layer processing stages of finishing, the following steps are repeated: stopping the machine at a process fault, removal of the operational disturbance, goods inspection on completion of the process, with removal of faults according to the best option. The effort devoted to this depends upon the type of process step and the value of the web. As a rule, but at least before delivery to the stage where tailored articles are produced, another goods inspection is carried out. In each subsequent goods inspection, the faults present and the additional ones must be reevaluated and removed if possible.
Of immediate importance is also the direct measurement of the change in the length of the web during finishing. The shrinkage or stretching of the textile surface is a significant parameter and a sensitive indicator for the consistency of many processes during finishing. A measurement of the current length of the web at any arbitrarily selected point in the process permits a series of improvements in processing precision.
A significant amount of scrap arises during the processing of the textile surface on the cutting table. With an efficient layout of the individual pieces, it is possible to keep the quantity of scrap to a minimum. This layout, according to the current state of the art, presupposes a textile web which is free of faults. If the textile surface contains a fault, various options arise, for taking it into account. If the cut is not adjusted, the spot with the fault must be discarded after cutting.
An entire series of parts thus becomes unusable, and the parts in question must be identified and found. It is also possible to submit the cutting with the fault to further processing and remove the finished product from the line to be sold as a second. Most widespread is that method where the faulty part of the web is cut out over its entire width. But if cutting is adjusted to leave the zone with the fault outside the area used, the quantity of scrap is significantly reduced. This option would be indeed feasible in the case of completely automated and computerized layout procedures, but requires precise information relative to the position of the fault in the textile web.